In the production of sheet steel blanks, it is familiar from DE 199 62 754 A1 to roll metal strips flexibly in such a way that areas of different thickness arise over the length of the metal strip, e.g., tailored rolled blanks, otherwise referred to as TRBs.
EP 2 111 937 A1 discloses a method for producing flat sheet steel blanks which vary in their thickness, intended in particular for the production of component parts for motor vehicles. A sheet steel blank with a varying thickness is prefabricated as a starting workpiece. The sheet steel blank is then partially reworked by stamping with a die, so that the thickness of the sheet steel blank, which already exhibits a variable thickness, is modified locally.
A rolling process and a rolling device for producing a metal strip having a varying thickness over its width are described in EP 2 208 555 B1. Groups of rollers are traversed by the metal strip in the direction of rolling. EP 2 208 555 B1 discloses a metal strip bent from its original direction of movement in a first pass along a surface of a rolling attachment involved in the pass rolling on the metal strip to bend the metal strip beyond its yield point. This would result in an advantageous structural change in the material. However, the bending in this case would be required to take place exactly at right angles to the metal strip, so that the resistance to expansion would be reduced, which would result in a light material flow in the width direction of the material displaced during the pass.
A rolling process for the formation of single-piece rolled material that is profiled in respect to its thickness, in which the source material is formed in the width direction by rollers penetrating into the source material to a different depth over the width of the rolled material, is disclosed in DE 101 13 610 C2. It is proposed in this case that forming of the source material takes place one area at a time, and that a thickness profile that is three-dimensional and freely selectable, both in the longitudinal direction and in the width direction, is formed by the defined overlapping of the forming areas. In an illustrative embodiment for this purpose, DE 101 13 610 C2 describes a rolling device, of which the rollers are arranged one after the other in the form of a triangle, the contact surfaces of the rollers complementing one another precisely in such a way that a closed impression is introduced into the sheet steel component. DE 101 13 610 C2 further proposes that profiled rollers and rollers that are in engagement via their surface could be utilized. DE 101 13 610 C2 in any event discloses as a profiled roller those rollers which produce a groove-shaped impression.
DE 197 48 321 A1 discloses a method for rolling metal sheets, in which the metal sheets in the width direction exhibit a thin part and a thick part, which are produced in a number of steps. Each rolling step has a set consisting of a convex roller and a flat roller, the convex roller exhibiting a convex part. The thin part is formed in accordance with the extension in the width direction through the convex part of the convex roller, and the thick part is formed in accordance with the extension in the length direction through another part that is formed as the convex part of the convex roller. Sheets of different thicknesses can thus be produced transversely to the direction of rolling, although this method is not suitable in order to produce sheets of different thicknesses and shapes in the direction of rolling.
A further method for producing sheet metal profiles of different thicknesses transversely to the direction of rolling is disclosed in WO 2014 975 115 A1. The thickening of the material in this case is produced by a number of pairs of rollers lying one after the other. This method is also only suitable in order to produce sheet metal profiles of different thicknesses transversely to the direction of rolling. However, sheet metal profiles of different thicknesses and contours in the direction of rolling cannot be produced.
The increasing use of light alloys, such as aluminum, for structural elements of motor vehicles gives rise to particularly high demands in respect of the joining technology used for connecting a structural element to other components or structural elements of a motor vehicle. The selection of mechanical or thermal joining processes (pressure welding and fusion welding) for the connection of two component parts to one another is familiar in the art. In the case of mechanical joining processes, a riveted connection can be selected, for example, self-pierce riveting, otherwise referred to as SPR. Two sections for connection are placed one on top of the other for this purpose. Both component parts are deformed by a rivet, and they are thus positively connected to one another. Resistance spot welding, otherwise referred to as RSW, is a familiar thermal joining process. The two sections for connection in this case are again placed one on top of the other, and they are held between two electrodes, which usually consist of copper. A current is applied between the two electrodes, so that the local area between the two electrodes is melted.
In the mechanical method of riveted connection, appreciably high forces must be applied in order to be able to permanently establish the connection, because the assistance of heat is absent. Signs of contact corrosion because of different materials, also appear, for example, if the rivet is made of steel and the connection sections are made of aluminum, which can have an adverse effect on the durability. In the resistance spot welding method, with regard to aluminum, very high welding currents must be applied in order to be able to achieve adequate melting, which in turn can lead to a high tip wear of the copper electrodes. This requires a high energy consumption with the associated anticipated high energy costs. The high heat input can have an adverse effect on the structure of the material with respect to its durability.
A further method for producing a materially integral connection is the friction point welding process, in which the welding point is refilled, e.g., refill friction stir spot welding, otherwise referred to as RFSSW. This is a welding process in which little heat is introduced into the material. Frictional heat is generated by a rotating tool in order to plasticize the material. Welds are thus produced at about 400-450° C. in the case of aluminum alloys and, as a result, hot crack formation and high hydrogen solubility are avoided in aluminum, which exhibits a melting temperature of approx. 660° C. A low-heat joining process can be utilized in the case of aluminum alloys, in order to be able to ensure a high quality of the joint.
On balance, the resistance spot welding method is the fastest in comparison with SPR and RFSSW in terms of the time taken to establish a connection. The weld quality offered by the resistance spot welding method is not particularly good, however, as a consequence of the complete melting of the joint in the case of aluminum alloys. Although a connection by SPR is faster than a connection by RFSSW, SPR is nevertheless very unattractive because of the very considerable cost of the rivets and the possibility of contact corrosion. Furthermore, the use of multiple rivets would be required depending on the thickness of the connection point. In this respect, the connection process of RFSSW represents a particularly good alternative to the connection process by SPR. This is true in particular if the connecting surfaces decrease in thickness. Although the time taken to establish the connection is more or less the same in both the SPR and RFSSW methods, harmful contact corrosion is absent in the method by RFSSW, and the particularly high material costs of the large number of rivets is also avoided.
A feature common to all the connection methods is that the connecting surface, that is to say a peripheral flange, for example, should be as small as possible, that is to say thin. The advantages of a connecting surface that is as thin as possible include, in addition to shorter welding/joining times, a low consumption of materials as well as the achievable reduction in weight, which is advantageous in particular in the automotive industry. A metal strip having different thicknesses in the longitudinal direction is, in fact, capable of being produced in the rolling process that is familiar from the prior art. It is known, furthermore, that the metal strip can also exhibit different thicknesses in the width direction. Three-dimensional thickness profiles are also possible in this case, as shown in DE 101 13 610 C2. Multiple passes and rollers arranged both behind each other and next to each other are necessary for this purpose, however, although complicated height and positional settings are still required. In light of the foregoing, room for further improvements can still be identified in this respect in methods for producing structural elements for motor vehicles.